[PECOFF] Exported name should match C++ mangled name

Currently you can omit the leading underscore from exported
symbol name. LLD will look for mangled name for you. But it won't
look for C++ mangled name.

This patch is to support that.

If "sym" is specified to be exported, the linker looks for not
only "sym", but also "_sym" and "?sym@@<whatever>", so that you
can export a C++ function without decorating it.